The invention relates to power units comprising the internal combustion engine, particularly a Diesel engine, supercharged by a turbocompressor unit comprising at least one compressor and at least one turbine receiving the engine exhaust gases, at least one by-pass pipe through which the air delivered by the compressor and which is not taken by the engine can flow towards the turbine, and throttle means in the by-pass pipe with a variable cross section and arranged to generate a pressure difference which is an increasing function of the pressure in the by-pass pipe and is substantially independent of the flow rate through the pipe. All the air not absorbed by the engine is then transmitted through the by-pass pipe from the compressor to the turbine.
The pressure difference provides for scavenging of the engine, which is essential in the case of two-stroke engines, which is necessary for cooling the valves and cylinder head in wide-bore four stroke engines and which is useful for discharging residual gases, all the more if the volumetric ratio of the engine is smaller. Since the pressure difference is independent of the flow rate (i.e. of the engine speed when the unit operates at a given power), the relation between the pressure ratio of the compressor and the expansion ratio in the turbine is independent of the engine speed and the compressor can be operated very near the surge line, in a high-efficiency region.
A unit of that kind is disclosed and claimed in the copending patent application Ser. No. 721,576 filed Sept. 8, 1976 as a division of patent application Ser. No. 437,748, filed Jan. 29, 1974 in the name of Jean Melchior, now U.S. Pat. No. 3,988,894 which describes, by way of example, throttle means comprising a throttle member which determines a constricted flow cross-section for air in the by-pass pipe and is subjected to a force proportional to the pressure difference and thus acts as a combined throttle and control means in cooperation with other means sensitive to the upstream or downstream pressure. The throttle member is not balanced.
A system of this kind is the easiest to build and is satisfactory in most cases. However, since the same element is used for control and throttle purposes, owing to the use of a non balanced throttle element, disadvantages occur in certain units and/or under certain operating conditions.
The throttle assembly consisting of the member and the associated control means constitute a mechanical oscillator. In some units and under certain operating conditions, the oscillator may resonate. Undesirable vibration of the throttle means may result in destruction thereof and the air stream flowing through it becomes unstable and may cause the compressor to surge and the flame in the combustion chamber to be blown out.
In addition, it is hardly possible to change the transfer function of that oscillator, since the control and throttle operations are combined, as will now be seen. The pressure drop .DELTA. P generated by the throttle means is substantially linear: EQU .DELTA. P = .alpha. P + .beta.
the coefficient .beta. can be modified by a small amount by adjusting a spring which exerts a biasing force on the control means. The coefficient .beta. can for instance be varied, by subjecting the spring to the pressure P upstream of the throttle means e.g. using a movable stop which modifies the return force of the spring and is moved in dependence on the pressure. On the other hand, it is difficult to adjust the coefficient .alpha. in dependence on an operating parameter of the power unit.
If the compressor has a complicated surge line (e.g. axial multi-stage compressors), the operating line of the turbo-compressor cannot be made to pass through the optimum efficiency points if the pressure drop is a linear function of the compressor output pressure. Last, the fact that the throttling and control actions are combined implies that the amount of actuating force is dependent on the transversal size of the throttle element. The forces acting on the control element cannot be increased except by increasing the dimensions of the throttle element, and the size may become prohibitive.